add nav_footer as a block region for footer elements

Currently the theme has a section in the footer to print $nav_footer. However, there is nothing in the theme settings that ever defines or even mentions nav_footer.

Here is a simple patch that adds it to the bluecheese.info file, where it should appear. this will allow themers to put either blocks of static content or even blocks from a special footer menu that we could create. For now in combination with my patch for #650128: bluecheese block grids on the fly?, the footer will support auto-detected 960.gs grid widths, for a listing of say 2,3, or 4 blocks.